1. Field of the Invention
This invention relates to an apparatus for driving a wheel of an automobile, and more particularly to a bearing unit for a wheel called a 3.5-generation hub unit in which a constant-velocity joint and hub unit are integrated into one unit. This apparatus is used for supporting a driven wheel that is supported by an independent-type suspension {front wheels for a FF car (front engine, front-wheel drive), rear wheels for a FR car (front engine, rear-wheel drive), rear wheels for a RR car (rear engine, rear-wheel drive) and all the wheels for a 4 WD car (4 wheel drive)} such that the driven wheel rotates freely with respect to suspension, as well as for rotating and driving the driven wheel.
2. Description of the Related Art
In order to support a wheel such that it can rotate freely with respect to the suspension, a wheel-support bearing unit, having an outer race and inner race that rotate freely by way of rolling members, is used. Moreover, in the independent-type suspension, the wheel-support bearing unit, which supports the driven wheel, is combined with a constant-velocity joint, and it must smoothly transmit the rotation of the drive shaft to the driven wheel (maintaining constant velocity) regardless of relative displacement between the differential gear and the driven wheel or the steering angle applied to the driven wheel. FIG. 3 shows a typical apparatus for the driven wheel of an automobile in which the wheel-support bearing unit 1 is combined with the constant-velocity joint 2.
Here, the wheel-support bearing unit 1 comprises an outer race 3 on the radially inside of which a hub 4 and inner race 5 rotate freely by way of a plurality of rolling members 6. Of these, the outer race 3 is fastened to the knuckle 8 (see FIG. 4) of the suspension by a first flange 7 that is formed around the outer surface of the outer race 3 such that it does not rotate during operation. In addition, there are rows of outer-ring raceways 9 formed around the inner peripheral surface of the outer race 3, and the hub 4 and inner race 5 are supported on the radially inside of this outer race 3 such that they are concentric with the outer race 3.
On the outside end (the outside end in the width direction of the automobile when the bearing unit is installed in the automobile, the left end in FIGS. 1 to 4) around the outer peripheral surface of the hub 4, there is a second flange 10 for supporting the wheel. Moreover, there is a first inner-ring raceway 11 formed around the outer peripheral surface in the middle of the hub 4, and likewise, the inner race 5 fits around a small-diameter section 12 that is formed on the inside end (the inside end in the width direction of the automobile when the bearing unit is installed in the automobile, the right end in FIGS. 1 to 4), and a second inner-ring raceway 13 is formed around the outer peripheral surface of the inner race 5. There is also a first spline hole 14 formed in the center of the hub 4.
On the other hand, the constant-velocity joint 2 comprises an outer ring 15 for the constant-velocity joint, an inner ring 16 for the constant-velocity joint, and a spline shaft 17. Of these, the outer ring 15 for the constant-velocity joint and the spline shaft 17 form a drive shaft member 18. In other words, this spline shaft 17 is formed on the outside end of this drive-shaft member 18, and it freely fits in the first spline hole 14 described above, and the outer ring 15 for the constant-velocity joint is formed on the inside end of the drive-shaft member 18. At a plurality of locations in the circumferential direction around the inner peripheral surface of this outer ring 15 for the constant-velocity joint, there are outside engaging grooves 19 which are formed such that they are each orthogonal with respect to the circumferential direction. Moreover, in the center of the inner ring 16 for the constant-velocity joint there is a second spline hole 20, and there are inside engaging grooves 21 that are formed such that they are each orthogonal with respect to the circumferential direction and are located around the outer peripheral surface of the inner ring 16 such that they correspond with the location of the aforementioned outside engaging grooves 19. There are balls 22 located between these inside engaging grooves 21 and outside engaging grooves 19, and they are supported by a retainer 23 such that they can roll freely along the engaging grooves 21, 19. The shape of each of the components of the aforementioned constant-velocity joint 2 are substantially the same as the well-known Rzeppa type constant-velocity joint, and is not essentially related to this invention, so a detailed description will be omitted here.
In the case of a constant-velocity joint 2 and the wheel-support roller-bearing unit 1 as described above, the spline shaft 17 is inserted into the first spline hole 14 in the hub 4 toward the outside from the inside (from right to left in the figure). Also, by screwing a nut 25 onto the male screw section 24 formed on the outside end of the spline shaft 17 on the part that protrudes from the outside end of the hub 4, and then tightening the nut 25, the hub 4 and the spline shaft 17 are fastened together. In this condition, the surface on the inside end of the inner race 5 comes in contact with the surface on the outside end of the outer ring 15 for the constant-velocity joint, so the inner race 5 does not move in a direction that would cause it to come apart from the small-diameter step section 12. At the same time, the rolling members 6 are properly pre-loaded.
Furthermore, when installed in the suspension of an automobile, the male spline 27 that is formed on the outside end of the drive shaft 26 makes a spline fit with the second spline hole 20 that is formed in the center of the inner ring 16 for the constant-velocity joint. A stop ring 29 that fits in the installation groove 28 that is formed all the way around the outer peripheral surface on the outside end of the male spline 27 fits in an anchoring step section 30 that is formed around the edge on the opening on the outside end of the second spline hole 20, and this prevents the male spline 27 from coming out from the second spline hole 20. The output shaft section of the differential gear, not shown in the figure, is provided with the trunnion of a tripod-type constant-velocity joint, also not shown in the figure, and the inside end of the drive shaft 26 is connected to the center of the trunnion. As the automobile moves, the drive shaft 26 rotates at constant rpm, however, a thrust load is repeatedly applied in both axial directions due to the resistance of the tripod-type constant-velocity joint that occurs during rotation.
In the wheel drive apparatus for an automobile described above and shown in FIG. 3, the wheel-support roller-bearing unit 1 and constant-velocity joint 2 are fastened together by screwing and tightening a nut 25 to the screw section 24, so the weight of the unit is large. In other words, it is necessary to lengthen the splint shaft 17 by the amount of the male screw section that is formed on the spline shaft 17 on the outside of the constant-velocity joint 2, as well as a nut 25 is necessary. Therefore, the dimension in the axial direction, as well as the weight, of the wheel drive apparatus for an automobile increases by the amount of the screw section 24 and nut 25.
In regards to this, as shown in FIG. 4, U.S. Pat. No. 4,881,842 discloses a more simple construction that makes it possible to fasten the wheel-support bearing unit and constant-velocity joint in a way such that the dimension in the axial direction is shortened and the weight is reduced. In this second example of prior construction shown in FIG. 4, the hub 4 is supported on the radially inside of the outer race 3, which is fastened to the knuckle 8, such that it can rotate freely by way of the rolling members 6 that are arranged in a plurality of rows. In addition, the spline shaft 17 of the drive-shaft member 18a fits in the first spline hole 14 that is formed in the center of the hub 4. There is an installation section 31 formed on the surface of the outside end of this spline shaft 17 for attaching a fitting tool for taking the spline shaft 17 into the first spline hole 14. Also, this spline shaft 17 is prevented from coming out of the hub 4 by a stop ring 33 that is fastened in an installation groove 32 formed around the outer peripheral surface on the tip end of the spline shaft 17. In this state, an elastic ring 34 is elastically compressed between the hub 4 and the outer ring 15 for the constant-velocity joint of the drive-shaft member 18a, to prevent the spline shaft 17 from rocking and moving inside the hub 4. In this second example of prior construction, the wheel drive apparatus for an automobile is made more compact and lighter by using the stop ring 33 for connecting the wheel-support bearing unit 1a and the constant-velocity joint 2a. 
In the case of the second example of prior construction described above, although the apparatus is made more compact and lighter, it is difficult to maintain adequate durability as is. The reason for this is explained below referring to FIGS. 3, 4.
When the wheel-drive apparatus for an automobile is operating and the driving power (torque) is transmitted while the center axes of the outer ring 15 and the inner ring 16 for the constant-velocity joint of the Rzeppa-type constant-velocity joint 2 (FIG. 3), 2a (FIG. 4) are not concentric (angle of intersection is not 180 degrees), then the force that acts on the balls 22, and the inside engaging grooves 21 and outside engaging grooves 19 of the constant-velocity joint 2 is no longer applied to the same horizontal surface orthogonal to the center axis. Therefore, a bending moment acts on the drive-shaft members 18 (FIG. 3), 18a (FIG. 4), and the drive shaft 26. Furthermore, in the case of unevenness in the pitch of the outside and inside engaging grooves 19, 21 due to manufacturing error, a component of load also occurs in the radial load direction as a resultant force of contact loads on the outside engaging grooves 19 and inside engaging grooves 21 and the balls 22. When a load component in the radial load direction occurs due to this kind of resultant force, the cross section of the drive-shaft members 18 (FIG. 3), 18a (FIG. 4) and the drive shaft 26 are not only equally loaded by a twisting torque and bending moment, but they are also loaded by a bending moment due to the aforementioned radial load that is multiplied with a distance in the axial direction from the balls 22 of the constant-velocity joint 2 (FIG. 3), 2a (FIG. 4).
In the case of the drive shaft 26, since the distance in the axial direction from the balls 22 of the constant-velocity joint 2 to the base end of the male spline 27, which is the weakest part, is short, so that the load due to the bending moment of the aforementioned radial load is small. On the other hand, in the case of the drive-shaft members 18 (FIG. 3), 18a (FIG. 4), the distance in the axial direction from the balls 22 of the constant-velocity joint 2 to the base end (inside end) of the spline shaft 17, which is the weakest part, is longer than the distance to the weakest point of the drive shaft 26. Therefore, the bending moment due to the aforementioned radial load becomes larger than the bending moment at the weakest part of the drive shaft 26. In addition, the effect of the load that acts between the outside and inside engaging grooves 19, 21 and the retainer 23 can also be taken into consideration, however, it is considered that the difference between the drive-shaft members 18 (FIG. 3), 18a (FIG. 4) and the weakest part of the drive shaft 26 in fatigue strength, is outstandingly affected by the bending moment due to the aforementioned radial load.
The inventors of this invention performed tests on a driven wheel apparatus for an automobile in which the wheel-support bearing units 1 (FIG. 3), 1a (FIG. 4), are combined with the constant-velocity joints 2 (FIG. 3), 2a (FIG. 4) to investigate which part received the most damage. In the test, various joint angles (supplementary angle of the intersecting axis angle) were applied to the constant-velocity joints 2 (FIG. 3), 2a (FIG. 4), and a xc2x11470 Nxc2x7m torque was repeatedly applied to the drive shaft 26. With the spline fitting area 35 on the hub side, where the first spline hole 14 fits with the spline shaft 17, and with the spline fitting area 36 on the joint side, where the second spline hole 20 fits with the male spline 27, the small spline diameter (male bottom diameter, female peak diameter) is 24 mm, and they form a module 1. The results of the test are shown in Table 1.
As can be clearly seen from the test results, in the case of using a nut 25, as shown in FIG. 3, to fasten the wheel-support bearing unit 1 with the constant-velocity joint 2, the spline fitting section 35 on the hub side and the spline fitting section 36 on the constant-velocity joint side are damaged at the same rate, and the number of cycles until failure was large (long life). Also, there was a case where the tripod-type constant-velocity joint was damaged before either of the spline fitting sections. On the other hand, when a stop ring 33, as shown in FIG. 4, is used to fasten the wheel-support bearing unit 1a with the constant-velocity joint 2a, the spline fitting section 35 on the hub side was damaged in all cases, and the number of cycles until failure was small (short life).
The reason that the life of the spline fitting section 35 on the hub side is short due to the difference in the connection of the wheel-support bearing units 1, 1a and constant-velocity joints 2 (FIG. 3), 2a (FIG. 4) is as follows. First, in the case of construction using a nut 25 as shown in FIG. 3, the surface on the outside end of the hub 4 and the inner surface of the washer 37 attached to the nut 25, and the surface on the inside end of the inner race 5 and the surface on the outside end of the outer ring 15 for the constant-velocity joint come in strong direct contact (friction engagement) with each other. As a result, the substantial part of the twisting torque or bending moment are supported by the respective contact surfaces, so that the twisting torque and bending moment applied to the base of the spline shaft 17 are reduced by that amount and the durability of this base is improved.
On the other hand, in the case of construction using a stop ring 33, as shown in FIG. 4, there is no strong contact (friction engagement) between the inside surface of the stop ring 33 and the surface on the outside end of the hub 4, and the surface on the inside end of the inner race 5 and the surface on the outside end of the outer ring 15 for the constant-velocity joint. The contact force is limited (small) by the elastic force of the elastic ring 34, so that the force of the friction engagement on the contact surface is small, and the amount that this portion supports the twisting torque and the bending moment is very limited. Therefore, most of the twisting torque and bending moment act on the base of the spline shaft 17. The cross-section of each surface is loaded by an uneven load, and when considering the size of the bending moment due to the aforementioned radial load, the base of the spline shaft 17 is loaded by a bending moment that is larger than the bending moment acting on the base of the male spline 27, that is formed on the outside end of the drive shaft 26, so when the shaft diameters are the same, the base of the spline shaft 17 is damaged first.
Taking the above problems into consideration, an object of this invention is to provide a wheel-drive apparatus for automobile that makes it possible to effectively maintain adequate durability even when the wheel-support bearing unit and constant-velocity joint are connected by a stop ring.